1. Field of the Invention
This invention relates to barium strontium .beta.-diketonates, processes for producing the same and processes for producing barium strontium-containing oxide dielectric films by the chemical vapor deposition method (hereinafter referred to simply as the CVD method) with the use of the same.
2. Description of the Prior Art
Investigations are in progress on (Ba,Sr)TiO.sub.3 films (hereinafter referred to simply as BST films) having high dielectric constants and ferroelectric (Sr,Ba)Nb.sub.2 O.sub.6 films (hereinafter referred to simply as SBN films) as DRAM capacitors usable in the generation of gigabit techniques and the CVD method excellent in mass productivity and step coverage properties is highly expected to be useful in forming these films. However, these films are three-component systems, which makes it difficult to regulate the compositional ratio Ba:Sr:Ti. This difficulty is partly caused by the fact that there has been known no Ba and Sr compounds usable as a CVD feedstock which is in the form of a liquid being excellent in the controllability in supply, has a sufficient vapor pressure and shows a high heat stability at the source temperature.
At the present time, investigations are in progress mainly on barium .beta.-diketonates, in particular, barium bis(dipivaloylmethanate) [hereinafter referred to simply as Ba(dpm).sub.2 ] as the barium compound, and strontium .beta.-diketonates, in particular, strontium bis(dipivaloylmethanate) [hereinafter referred to simply as Sr(dpm).sub.2 ] as the strontium compound. These compounds have melting points in the range of 200 to 220.degree. C. and vapor pressures at 200.degree. C. of about 0.01 Torr.
It is much convenient to control the supply system from two sources compared with the case of the supply from three sources. Although the source number can be lessened by using as one source a single compound containing both of two components Ba and Sr, no such compound has been known hitherto. It is anticipated that a metal .beta.-diketonate composed of two different elements might be formed as in the case of a double alkoxide formed from two metal alkoxides of different elements which are liable to undergo association with each other. However, there has never been reported in public so far that such a molecule is formed. J. E. Schwarberg, R. E. Sievers and R. W. Moshier [Anal. Chem., 42, 1828 (1970)] reported that when a mixture of Ca(dpm).sub.2 with Sr(dpm).sub.2 was subjected to gas chromatography at 250.degree. C., only one peak was observed and this peak showed a retention time different from those of Ca(dpm).sub.2 and Sr(dpm).sub.2 chromatographed individually; that thus Ca(dpm).sub.2 could not be separated from Sr(dpm).sub.2 by gas chromatography; and CaSr(dpm).sub.3.sup.+ was observed in the mass spectrum of a fraction obtained by melting Ca(dpm).sub.2 together with Sr(dpm).sub.2 and recovering by evaporation. Based on these facts, they estimated that Ca(dpm).sub.2 would react with Sr(dpm).sub.2 in the course of the gas chromatography to thereby form a composite complex CaSr(dpm).sub.4 with a heterogenous elemental nucleus. EQU [Ca(dpm).sub.2 ].sub.2 +[Sr(dpm).sub.2 ].sub.2 =2CaSr(dpm).sub.4.
However, the identification of CaSr(dpm).sub.4 and measurement of its physical properties are insufficient and still at the level of estimation. As a matter of course, no description on (Ba,Sr).sub.1 (dpm).sub.2 is given therein.